The file attachment caption editor didn't show any buttons, which was

confusing to users. The problem was that there wasn't a great place to

put the buttons.
This change fixes this by adding a section just below the editor. The

buttons appear on a greyish background, and sort of just appear below

the thumbnail, overlapping any thumbnails that may be below it. The

reason it overlaps it to prevent any shifting/staggering of other

thumbnails.
Some work had to be done to properly set offsets and to change sizes of

the caption elements, in order to cleanly overlap anything below.

The sizes are set so that the overlap completely obscures the header

text from a thumbnail below, which is nicer-looking than ony

partially-obscuring the text.

Tested with several thumbnails.
Tested that the editor text lined up properly with the original text, creating a seamless transition.
I checked this on Chrome and Firefox.